This invention relates to an integrated antenna catheter or probe which relies on electromagnetic radiation to simultaneously controllably heat, and detect the temperature of, fluid or tissue adjacent to the probe. By placing the probe at the region of interest in the body, one can detect, diagnose and treat certain abnormalities associated with tumors, cardiac arrhythmias, benign prosthetic hyperplasia (BPH) and the like. When placed in a patient's vascular system, the catheter or probe can be used to measure temperature or even to raise tissue temperature during heart surgery. It relates especially to an improved probe of the type described in the above application Ser. No. 11/474,883, the entire contents of which are hereby incorporated herein by reference.
Referring first to FIGS. 1 to 3 of the drawings, the catheter or probe 10 described in the above application includes an inner conductor 16 and a coaxial tubular outer conductor 18. The distal or leading end 16a of conductor 16 is connected to the center of a conductive discoid toe plate 22 spaced in front of the outer conductor 18 which space is filled by a discoid dielectric spacer member 24. A hemispherical conductive shell 26 is mounted to the distal face of toe plate 22. Together they form a conductive distal end or tip 10a of probe 10. Shell 26 also defines a fluid-tight space 28 between the shell wall and the toe plate.
The proximal or trailing end of outer conductor 18 is closed by a discoid heel cap 30 connected to conductor 18 and to the proximal end 16b of inner conductor 16 which end extends into an opening 32 at the center of heel cap 30. The proximal end of center conductor 16 is also connected to the distal end of an inner conductor 33 of cable 14. Those two conductors meet in opening 32 with the cable end being anchored to heel cap 30.
The segment of inner conductor 16 within the outer conductor 18 carries a dielectric sleeve 34 and is supported within conductor 18 by a conductive insert or carrier 36 which fits in, and extends the length of, conductor 18, thus forming a coaxial transmission line. The conductor 16 and its sleeve 34 extend along an axial passage 38 in the insert. Insert 36 is in electrical contact with both outer conductor 18 and heel cap 30.
Still referring to FIGS. 1 and 2, a sheath 52 of a dielectric material surrounds the outer conductor 18 of catheter 10. However, that sheath does not extend all the way to the distal end of the conductor, but rather terminates at a selected distance therefrom. The proximal end of sheath 52 blends into cable 14.
A filter circuit 54 and a microwave radiometer circuit 56, arranged in one or more monolithic microwave integrated circuit chips (MMICs), are mounted to the top of insert 36. Also, mounted directly to the inner conductor 16 just ahead of insert 36 is a coupling capacitor 58 which is recessed into the spacer member 24. One terminal of capacitor 58 is connected electrically to conductor 16 and the other is connected by way of a lead (strip or wire) 60 to the first circuit 54 which is, in turn, connected to circuit 56. The output signal from the last circuit 56 as well as certain bias and control voltages are carried on a conductor group 64 which extends along the top of insert or carrier 36 and exits the catheter through a hole 66 in heel cap 30. There, those conductors join corresponding conductors 68 (FIG. 2) which extend along cable 14 to an external control unit (not shown). Also, a ground return conductor 69 from circuit 56 connects to a corresponding conductor 70 in cable 14.
Preferably the radiometer operates at a frequency in the microwave range. A conventional Dicke-type microwave radiometer is disclosed in my U.S. Pat. No. 4,557,272. Similar radiometer designs on a chip are available from Meridian Medical Systems, LLC, the assignee of this application.
Referring now to FIG. 1, basically the inner conductor 16 in catheter 10 comprises an RF coaxial transmission line terminated by the conductive rounded tip 10a. The transmission line is operated at the output signal frequency of a remote transmitter is (not shown). When the transmitter is operative, the transmission line will radiate energy for heating only from the uninsulated segment of the catheter between the catheter tip 10a and the distal end of the dielectric sheath 52. Thus, that segment constitutes a RF heating or transmitting antenna T whose length is determined by the forward or distal extent of sheath 52 on outer conductor 18. In other words, increasing the length of sheath 52 will reduce the exposed length of conductor 18, i.e. the surface that could contact tissue, and, in turn, will reduce the antenna T length. Since the outer conductor 18 is at the same RF potential as conductor 16, it can provide an RF path between the antenna T and the transmitter.
Referring to FIGS. 1 and 3, the conductive catheter tip 10a also comprises a temperature sensing microwave receiving antenna R which can pick up thermal emissions at depth from tissue adjacent to the catheter 10. The segment of conductor 16 from the tip 10a to its junction with capacitor 58 comprises the microwave receiving path and this path continues along the lead strip 60 to filter circuit 54 and thence to radiometer circuit 56. It should be noted that while conductor 33 is basically an extension of conductor 16, it conducts only the RF signal via outer conductor 18, while conductor 16 conducts both the RF and microwave signals. Thus, the probe's antennas T and R are contained in a common structure and basically constitute a single dual frequency antenna.
To enable catheter 10 to simultaneously heat (transmit) and detect temperature (radiometrically sense), a passive diplexer D is integrated into catheter 10 in order to block the transmitter signals from the microwave receiving path and isolate the microwave signals from the signal path from the transmitter. The diplexer D is formed by the filter circuit 54 coupled with lead 60 and capacitor 58 along with a quarter-wave (λR/4) shorted stub S (FIG. 1) constituted by the segment of catheter 10 extending from the connection of capacitor 58 to conductor 16, to the heel cap 30. This quarter wave stub S should be tuned to the frequency of the radiometer, thus providing a low loss path to circuit 56. The diplexer D also includes a high pass filter in at least one of circuits 54, 56.
The tuned length of the stub S, i.e. the catheter segment between capacitor 58 and heel cap 30, is determined by the dielectric constant of the material in sleeve 34 as well as the radiometer frequency. For example, at a radiometer frequency of 4 GHz, when sleeve 34 is of PTFE (K=2.1), a suitable stub length may be 0.5 inch. On the other hand, when a K=9 material is used, the stub length may be reduced to 0.25 inch. For an intermediate length, e.g. 0.38 inch, a K=3.8 material may be used.
For the minimally invasive catheter of interest here, it is essential that the length of stub S be as short as possible. This, in turn, requires that the sleeve 34 material have an especially high dielectric constant, i.e. K=9 or more. Only hard ceramics such as alumina (K=9.8) meet this criterion.
In practice, we have found that it is quite difficult to reliably manufacture at a reasonable cost a thin-wall, e.g. 0.005 in, dielectric sleeve 34 of alumina ceramic. Such sleeves are quite fragile and difficult to make on a high volume basis. Therefore, it would be desirable to be able to provide a probe of the above type which can be made and marketed on a competitive basis with prior medical probes used for this general purpose.